Crane types, terminology, capacity and configuration
Knowing the machine
A slewing mobile crane is one that can rotate (slew) about a vertical axis and travel between sites. Common configurations include truck-mounted, rough-terrain, all-terrain and pick-and-carry types. Key terms: boom (telescopic or lattice), jib, fly jib, counterweight, outriggers, slew ring, hoist rope, hook block, head sheave, load moment indicator (LMI) or rated capacity limiter (RCL), and the load chart.
Capacity depends on radius, boom length, boom angle, counterweight, outrigger configuration (fully extended, part extended, retracted) and the load chart for that specific crane. Rated capacity is the maximum load the crane may lift in a given configuration; gross capacity is the load plus all lifting gear, hook block and any other suspended items. Never confuse the two. Deductions for lifting gear, hook block and jib must be made before comparing to the chart.
Traps in exam questions
- Using the chart for the wrong configuration (e.g., outriggers fully extended when they are not).
- Forgetting to deduct the rigging and hook block from the rated capacity.
- Assuming a longer boom always means more capacity; usually it means less at the same radius.
- Mixing radius definitions: radius is measured horizontally from the slewing centre to the centre of gravity of the load, not to the boom tip.
- Ignoring the LMI/RCL warning or override - never override a safety device.
Worked example
A crane chart shows 8.0 t at 10 m radius with full outriggers. The hook block and slings weigh 0.6 t. The maximum payload is 8.0 − 0.6 = 7.4 t. A 7.6 t load is therefore overloaded even though it is below the chart figure.
Exam tip
Always check the configuration line on the chart before reading a number.